The present invention relates to a method for radiosynchronization of base stations in a simulcasting network which includes a plurality of base stations (BS) in a synchronization area, i.e. base stations that must be synchronized to each other.
As to the general description of the simulcasting paging network, reference is made to European patent application EP-0197556, which shows a prior art method for the radio synchronization of base stations in a paging network.
Simulcasting paging networks operate in a quasi synchronous manner. Several transmitters of base stations transmit the same information simultaneously in order to achieve large and continuous coverage. A problem is in that a paging receiver may be located within the coverage areas of two transmitters. Since transmitters operate on the same frequency, they interfere with each other unless synchronized.
The purpose of synchronization of the paging base stations is to get the base stations to transmit the same information at exactly the same instant (so-called quasi synchronous transmission). In wide area paging systems with digital information, this means that the same information symbol (a data bit) is transmitted from various base stations at exactly the same time. According to one of the standards, for instance, transmission is quasi synchronous if the phase difference of symbols transmitted by various base stations, upon the arrival thereof in a paging receiver, does not exceed 1/4 of the time required by transmission of the symbol.
As transmission speed increases, the requirement for synchronization will be stricter since the duration of a symbol becomes shorter.
For instance in a wide area paging system there may be set a practical requirement that no more than .+-.10 microseconds difference can be accepted in timing of transmissions from adjacent base stations. This requirement of accuracy could be met by high-precision time reference (atomic clock), which is synchronized to a certain time reference, or with continuous reception of time from a high-precision time reference. Both of these are far too expensive. Therefore a preferred solution is to provide a base station with a quartz oscillator as time reference and to synchronize the clocks of the base stations periodically to each other by using radio path for the transmission of synchronization signal.
The preferred embodiment of the method according to the present invention resembles the prior art method according to EP-0197556 in that the base stations receive from a common controller a synchronization plan, which includes selection of base stations for sending the sync message in a given order and at predefined times. In the prior art method the synchronization plan includes also a predefined route along which the synchronization propagates from one base station to the other. In other words, when a base station is sending the sync message, there is a predefined base station which synchronizes its clock to the clock of the sending base station, whereafter the predefined base station starts sending the sync message. This predefined route of synchronization causes, however, some drawbacks in that the synchronization plan becomes complicated and the synchronization is sensitive to errors. One disturbance in reception of the sync message results in erroneous synchronization.